Motor control unit for electrically operated power steering devices

ABSTRACT

An electrically operated power steering device connected to a steering system of a motor vehicle to provide supplementary steering power by means of an electric motor. A motor control unit for the electrically operated power steering device controls the rotational direction and output torque of the electric motor by the sum of an assistance signal based on the signal from a torsional torque sensor and a velocity sensor in the steering system, a restoration signal based on the output signal from a steering angle sensor, and an auxiliary assistance signal based on the output signal from the torsional torque sensor in the steering system as a command signal.

BACKGROUND OF THE INVENTION Related Application

This application is a continuation of our co-pending application Ser.No. 785,929, filed Oct. 9, 1985, now abandoned.

The present invention concerns a motor control unit for electricallyoperated power steering devices which provide supplementary steeringpower to vehicle steering systems by driving an electric motor.

Previously, there has been known, from the Japanese publication ofexamined patent application (JB, B1) No. 45-41246 (1970), a motorcontrol unit for electrically operated power steering devices, in whicha torsional torque sensor is provided to measure the torsional torque ofthe steering shaft on completion of a turn, the rotational direction andoutput torque of the electric motor being controlled according to theoutput signal from said torsional torque sensor.

In electrically operated power steering devices such as mentioned above,since there exist reduction gears in the transmission from motor to thesteering mechanism, the restoring action of the steering wheel oncompletion of a turn tends to reduce due to friction in the reductiongears and inertia of the electric motor. The increased motor inertiaalso results in a delay motor response to the steering input, andconsequently in the feel of the steering. If the power steering deviceis designed to provide sufficient supplementary steering power (powerassistance) at low speeds, the assistance would become excessive at highspeeds, making the steering wheel feel light and insecure. Also, sincethere occurs a time lag between the steering input and generation of thepower assistance due to a signal phase lag in the motor control circuitbetween the torsional torque sensor and the electric motor, the controlsystem is sensitive to disturbances, and prone to self-excited vibrationwhen steering while the vehicle is at a standstill.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a motor control unitfor electrically operated power steering devices, which has improvedsteering wheel restoration characteristics on completion of a turn, hasimproved motor response to steering input, avoids excessive powerassistance at high speeds, and prevents self-excited vibrations whensteering at a standstill.

To meet the objectives mentioned above, according to the presentinvention, an electrically operated power steering device in which therotational direction and output torque of the electric motor arecontrolled by a command signal based on the output signal from atorsional torque sensor which measures the torsional torque in thesteering system, is further provided with a velocity sensor to measurethe vehicle speed and a steering angle sensor to measure the angulardisplacement of the wheel. Also provided are: an assistance command unitwhich produces an assistance signal which, based on the output signalsfrom said torsional torque sensor and velocity sensor, increases inabsolute value with increasing torsional torque and decreases inabsolute value with increasing vehicle speed; a restoration command unitwhich produces a restoration signal based on the output from saidsteering angle sensor; and a phase compensation command unit whichimmediately sends out an auxiliary assistance signal corresponding tothe rate of change of output from said torsional torque sensor. Therotational direction and output torque of the electric motor iscontrolled by the sum of said assistance, restoration and auxiliaryassistance signals as the command signal.

Provided such means, the command signal controlling the rotationaldirection and output torque of the electric motor for power assistance,diminishes in magnitude as the vehicle speed increases, and assumes avalue causing the electric motor to generate an output torque tending toreturn the wheel toward neutral position when the input steering forceis reduced and the absolute value of the torsional torque falls below avalue which depends on the steering angle. The motor starts upsimultaneously with steering input, as the auxiliary assistance signalacts as the command signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the components of an embodiment of thepresent invention.

FIG. 2 is a graph showing the output characteristic of the torsionaltorque sensor.

FIG. 3 shows the basic characteristic of the assistance signal iT.

FIG. 4 shows the characteristic of the additional constant signal Sv.

FIG. 5 shows how the assistance signal characteristic varies with speed.

FIG. 6 shows the characteristic of the multiplying factor signal.

FIG. 7 shows the characteristic of the restoration signal i θ.

FIG. 8 shows the characteristic of the auxiliary assistance signal ia.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention shall now be described by makingreference to the attached drawings.

In FIG. 1, electric motor for providing power assistance, which suppliespower assistance to the steering system through an unshown rack-pinionmechanism to which it is connected by reduction gears and joints, isindicated by reference number 1. The rotational direction and outputtorque of this motor is controlled based on a control signal by a drivecontrol unit 2 which includes a sign discriminator 21, an absolute valuetransformation unit 22, a duty ratio controller 23, a rotor currentdetector 24 and a motor driver 25. The control signal is fed to the signdiscriminator 21 and to the absolute value transformation unit 22, theoutput from the sign discriminator 21 in turn being fed to the motordriver 25 to switch the direction of motor current according to the signof the control signal, and the output from the absolute valuetransformation unit 22 being fed to the duty ratio controller 23 todetermine the duty ratio which in turn is fed to the motor driver 25 togenerate an output torque in accordance with the magnitude of thecontrol signal. The output torque of the electric motor 1 is controlledso as to converge toward the required value by measuring the rotorcurrent in the motor 1 with the rotor current detector 24 and feedingthe measured value back to the duty ratio controller 23.

According to the present invention, the control signal for the drivecontrol unit 2 is supplied by an assistance command unit 3, arestoration command unit 4 and a phase compensation command unit 5.

The assistance command unit 3 basically generates an assistance signalcorresponding to the magnitude and direction of the torsional torque inthe steering system. It consists of a torsional torque sensor 31connected to an unshown steering shaft to measure the direction andmagnitude of the torsional torque being applied, and an assistancetorque designator 32 which generates an assistance signal iT ofcharacteristic as shown in FIG. 3, in response to the output signal fromthe torsional torque sensor 31 whose characteristic is as shown in FIG.2. The characteristic of the assistance torque designator 32 is suchthat no output occurs while the magnitude of applied torque is below acertain level, the magnitude and sign of the output changing inaccordance with the magnitude and direction of applied torque when thisvalue is exceeded. Also provided in the assistance command unit are, avelocity sensor 33 which measures the vehicle speed, an additionalconstant generator 34 which, based on the output from the velocitysensor 33, generates an additional constant signal Sv of characteristicas shown in FIG. 4 which decreases in magnitude with increasing vehiclespeed, and an adder 35 which accepts said additional constant signal Svand the output of the torsional torque sensor 31 as input and whichsends its output to the assistance torque designator 32. The additionalconstant signal Sv is either added to or subtracted from the output oftorsional torque sensor 31 depending on the sign of the latter, to shiftthe characteristic curve shown in FIG. 3 to the left or to the right,with vehicle speed as a parameter. In other words, the output signalfrom the assistance torque designator 32 decreases in absolute value asvehicle speed increases under a given input torque, and increases inabsolute value with an increase in absolute value of input torque undera given speed. (See FIG. 5.) Further, a multiplying factor generator 36and a multiplier 37 are provided in the assistance command unit, tomodify this output as shown by the broken lines in FIG. 5 according tovehicle speed. On the basis of the output from the velocity sensor 33,the multiplying factor generator 36 generates a multiplier signal ofcharacteristic as shown in FIG. 6, which takes the value 1 at zerovehicle speed and asymptotically approachse 0 with increasing vehiclespeed. The multiplier 37 multiplies the output from the assistancetorque designator 32 by the multiplying factor, so that the assistancesignal from the multiplier 37 would take the form shown by the brokenlines in FIG. 5, according to speed.

The restoration command unit 4 generates a signal according to theangular displacement of the wheel to cause the steering wheel to returnto neutral position. It comprises a steering angle sensor 41, whichmeasures the angular displacement of the wheel in terms of the lateraldisplacement of the rack in an unshown rack-pinion mechanism, and arestoration torque designator 42 which generates a restoration (returntorque) signal iθ of characertistic as shown in FIG. 7 based on theoutput from said steering angle sensor 41.

The phase compensation command unit 5 consists of a phase compensator 51which accepts the output signal from the torsional torque sensor 31 toproduce an output proportional to its rate of change with respect totime, and a phase compensation designator 52 which produces an auxiliaryassistance signal ia of characteristic as shown in FIG. 8 for example,based on the output signal from the phase compensator 51. In theembodiment described, the output signal from the phase compensator 51 isalso added to the output from the torsional torque sensor 31 to be fedto the adder 35.

The assistance signal iT from the assistance command unit 3, therestoration signal iθ from the restoration command unit 4 and theauxiliary assistance signal ia from the phase compensation command unit5 are summed and fed to the drive control unit 2 as command signals.

Due to such construction, when a torsional torque occurs in the steeringsystem by operation of the steering wheel, the torsional torque sensor31 detects this torque and produces an output signal, upon which anassistance signal is generated by the assistance command unit 3, tocontrol the rotational direction and output torque of the electric motor1 through sign discriminator 21 and duty ratio controller 23 accordingto the absolute value of the assistance signal iT. The relationshipbetween the torsional torque and the assistance signal iT is basicallyas shown in FIG. 3. For example, in response to a rightward steeringinput torque, a positive assistance signal is generated, which increasesin magnitude with an increase in torque. Thus, the electric motor 1produces an output torque of magnitude corresponding to the appliedinput torque in the direction that assists rightward steering to relievethe steering load on the operator of the vehicle. In the case of aleftward turn the electric motor 1 is controlled to assist leftwardsteering under a negative assistance signal, to act similarly as in thecase of rightward steering.

According to the present invention, the functional relationship betweenthe torsional torque and the assistance signal is varied by the outputfrom the velocity sensor 33. For example, in FIG. 5 which shows thecharacteristics of the assistance signal in response to rightwardsteering input torque, where the condition at zero speed is indicated byM_(o), the characteristic curves shift along the X axis to the right toM₁ and M₂ as vehicle speed increases to V₁ and V₂ due to the addition ofthe additional constant, and the slopes of the curves are reduced fromM₁ to m₁ and from M₂ to m₂ due to multiplication by the multiplyingfactor. Hence, under given input torque, the magnitude of the assistancesignal decreases with the increase in vehicle speed. In other words, theoutput torque from the electric motor 1 decreases with an increase inspeed, so that excessive power assistance is avoided at high speedswhile providing sufficient power assistance at a standstill or at lowspeeds, thus providing positive feel to the steering system.

Meanwhile, the steering angle sensor 41 detects the angular displacementof the wheel following a steering input, and based on this signal, therestoration command unit 4 produces a restoration signal iθ ofcharacteristic as shown in FIG. 7, which is proportional to the steeringangle for steering angles within ±θ_(O) from neutral position, andconstant for steering angles exceeding θ₀ in magnitude, the signal beingnegative for region of rightward steering angle and positive for regionof leftward steering angle. Thus for example, when the right steeringangle θ₁ is maintained, the electric motor 1 is controlled by the sum ofthe positive assistance signal iT based on the positive output from thetorsional torque sensor 31 as mentioned before, and the negativerestoration signal iθ₁ based on the output from the steering anglesensor 41. In FIG. 3 where the assistance signal iT is indicated by thesolid line, the sum of the signals mentioned above would become asindicated by the broken line. Therefore, if the maintenance of the rightsteering angle θ₁ is released, the sum of signals would immediately takea negative (leftward) value along the broken line due to the rapiddecrease in torsional torque T. Thus, a leftward steering torsionaltorque would occur at the electric motor 1, to overcome friction in thereduction gears and the motor's moment of inertia. Hence the restoratingaction of the steering wheel is improved, so that the steering systemreturns smoothly to neutral position under a caster effect while thevehicle is in motion. As the restoration signal iθ diminishes inmagnitude toward zero with decrease in steering angle θ, the outputtorque of the electric motor 1 would totally vanish at or near theneutral position, thus improving convergence in hands-off condition athigh speeds.

The steering operation while the vehicle is at a standstill shall now bedescribed. In this case, due to the existence of large resistance, thetorsional torque would build up rapidly following a steering input,leading to a rapid increase proportional to torsional torque in outputfrom the torsional torque sensor 31. The rapid increase in torsionaltorque is detected by the phase compensator 51 of the phase compensationcommand unit 5, resulting in an output signal corresponding to the rateof change in torsional torque being added to the output signal from thetorsional torque sensor 31. Therefore, even in the very early stages ofa steering operation when the torsional torque T has not yet built up toa value large enough to produce an assistance signal iT, the assistancesignal iT would nevertheless be outputted immediately if the rate ofincrease in torsional torque is large enough, so that the electric motor1 would respond without delay upon exertion of a steering force at astandstill, thus preventing occurrence of self-excited vibration.

The auxiliary assistance signal ia, of characteristic as shown in FIG.8, is produced by the phase compensation designator 52 based on theoutput from the phase compensator 51. The auxiliary assistance signal isproportional to the rate of change with respect to time of torsionaltorque up to a certain magnitude, above which it takes constant values.Hence, in cases such as repeated hard steering alternately to left andright, the auxiliary assistance signal ia is generated immediately toabsorb the motor inertia, and to improve the feel of steering.

Although the additional constant generator 34 has been selected toproduce an additional constant signal which decreases with increasingspeed in the embodiment described above, it may also be selected toproduce an additional constant signal which increases with speed, inwhich case the additional constant signal would be subtracted from theoutput of the torsional torque sensor 31 at the adder 35.

In this respect the term "sum" in the claims also generically includessubtraction.

It is also possible for reduction in assistance signal withoutincreasing speed to be made with either the additional constantgenerator 34 or the multiplying factor generator 36 of the assistancecommand unit 3.

As described above, according to the present invention, application ofexcessive steering power at high speeds, which cause lightness of thesteering wheel and sense of insecurity, is avoided, since the commandsignal which controls the rotational direction and output torque of theelectric motor for power assistance, decreases in magnitude withincreasing speed. The restoring action of the steering wheel is alsoimproved, because the command signal takes a value causing the motor torotate in the restoring direction when the torsional torque falls belowa predetermined value which varies with the angular displacement of thewheel. Furthermore, the response of power assistance is improved due tothe occurrence of an auxiliary assistance signal immediately uponapplying a steering input, preventing self-excited vibration whensteering while the vehicle is at a standstill, and also absorbing themotor inertia on repeated steering in both directions to improve thefeel of steering.

What is claimed is:
 1. An electric power steering system for a motorvehicle having a motor operatively connected to a steering system forreducing steering effort, comprising:means comprising a torsion torquesensor for detecting torsion torque applied to the steering system in asteering operation for producing a torsion torque signal; meanscomprising a vehicle speed sensor for producing a vehicle speed signal;first means responsive to the torsion torque signal and to the vehiclespeed signal for producing an assist torque signal the absolute value ofwhich increases with increase of the absolute value of the torsiontorque signal and decreases with increase of the vehicle speed signal;means comprising a steering angle sensor for detecting steering anglefor producing a steering angle signal; second means responsive to thesteering angle signal for producing a return torque signal independentof vehicle speed for returning the steering system to a neutral state;third means for adding the return torque signal to the assist signal;and fourth means responsive to summation of the assist torque signal andthe return torque signal for producing a signal for driving the motor,whereby assist torque increases with increase of the absolute value ofthe torsion torque signal and decreases with increase of the vehiclespeed signal, and returned torque is produced independent of the vehiclespeed.
 2. The system according to claim 1, further comprisingmeanscomprising a multiplying factor generator for generating a multiplyingfactor signal based on said vehicle speed signal, said multiplyingfactor signal diminishes in magnitude as the vehicle speed increases,and said means comprising said multiplying factor generator multipliesthe assist torque signal by the multiplying factor signal.
 3. The systemaccording to claim 1, whereinsaid second means produces said returntorque signal constant for absolute values of the steering angle greaterthan a predetermined value and as a proportional linear function of thesteering angle for all other steering angles.
 4. An electric powersteering system for a motor vehicle having a motor operatively connectedto a steering system for reducing steering effort, comprising:meanscomprising a torsion torque sensor for detecting torsion torque in thesteering system in a steering operation for producing a torsion torquesignal; means comprising a vehicle speed sensor for producing a vehiclespeed signal; first means responsive to the torsion torque signal and tothe vehicle speed signal for producing an assist signal; meanscomprising a steering angle sensor for detecting steering angle forproducing a steering angle signal; second means responsive to thesteering angle signal for producing a return torque signal havingreverse polarity of the assist signal; third means for adding the returntorque signal to the assist signal; and fourth means responsive tosummation of the assist signal and the return torque signal forproducing a signal for driving the motor; means comprising an additionalconstant generator for generating an additional constant signal based onsaid vehicle speed signal, and wherein said first means includes anassistance torque designator receiving the sum of said torsional torquesignal and said additional constant signal.
 5. The system according toclaim 4, whereinsaid additional constant generator produces saidadditional constant signal decreasing with increasing vehicle speed. 6.An electric power steering system for a motor vehicle having a motoroperatively connected to a steering system for reducing steering effort,comprising:means comprising a torsion torque sensor for detectingtorsion torque in the steering system in a steering operation forproducing a torsion torque signal; means comprising a vehicle speedsensor for producing a vehicle speed signal; first means responsive tothe torsion torque signal and to the vehicle speed signal for producingan assist signal; means comprising a steering angle sensor for detectingsteering angle for producing a steering angle signal; second meansresponsive to the steering angle signal for producing a return torquesignal having reverse polarity of the assist signal; third means foradding the return torque signal to the assist signal; and fourth meansresponsive to summation of the assist signal and the return torquesignal for producing a signal for driving the motor; a phasecompensation command means comprising means comprising a phasecompensator for generating an output signal proportional to rate ofchange with respect to time of an output signal from said torsionaltorque sensor, and means comprising a phase compensation designator forproducing an auxiliary assistance signal based on the output signal fromsaid phase compensator.
 7. The system according to claim 6, whereintheoutput signal from said phase compensator is added to said torsiontorque signal.
 8. The system according to claim 7, whereinsaid thirdmeans adds said auxiliary, assistance signal to said assist signal andsaid return torque signal.
 9. The system according to claim 8,whereinsaid first means produces said assist signal responsive to thevehicle speed signal and sum of said torsion torque signal and saidoutput signal from said phase compensator.
 10. The system according toclaim 7, whereinsaid first means produces said assist signal responsiveto the vehicle speed signal and sum of said torsion torque signal andsaid output signal from said phase compensator.
 11. The system accordingto claim 6, whereinsaid auxiliary assistance signal is proportional tosaid rate up to a predetermined magnitude and has constant values abovesaid predetermined magnitude.
 12. The system according to claim 11,whereinsaid third means adds said auxiliary assistance signal to saidassist signal and said return torque signal.
 13. The system according toclaim 6, whereinsaid third means adds said auxiliary assistance signalto said assist signal and said return torque signal.
 14. An electricpower steering system for a motor vehicle having a motor operativelyconnected to a steering system for reducing steering effort,comprising:means comprising a torsion torque sensor for detectingtorsion torque applied to the steering system in a steering operationfor producing a torsion torque signal; means comprising a vehicle speedsensor for producing a vehicle speed signal; first means responsive tothe torsion torque signal and to the vehicle speed signal for producingan assist torque signal the absolute value of which increases withincrease of the absolute value of the torsion signal and decreases withincrease of the vehicle speed signal; means comprising a steering anglesensor for detecting steering angle for producing a steering anglesignal; second means responsive to the steering angle signal forproducing a return torque signal for returning the steering system to aneutral state; third means for adding the return torque signal to theassist signal; and fourth means responsive to summation of the assisttorque signal and the return torque signal for producing a signal fordriving the motor, whereby assist torque increasing with increase of theabsolute value of the torsion torque signal and decreasing with increaseof the vehicle speed signal is produced; the first means comprising anadditional constant generator for generating an additional constantsignal based on said vehicle speed signal; and wherein an assistancetorque designator receiving the sum of said torsional signal and saidadditional constant signal.
 15. The system according to claim 14,whereinsaid additional constant generator produces said additionalconstant signal decreasing with increasing vehicle speed.
 16. In anelectric power steering system having a steering wheel, a motoroperatively connected to a steering system for reducing steering effortof turning the steering wheel, torsion torque sensing means fordetecting torsion torque and direction of torsion applied on thesteering wheel in a steering operation and producing ouput signalsdependent thereon, vehicle speed sensor for producing an output signalcomprising a vehicle speed signal dependent on vehicle speed, and assistcommand signal generating means for generating an assist signal toassist the turning of the steering wheel responsive to output signals ofthe torsion torque sensing means and vehicle speed sensor, theimprovement comprisingdetecting means for detecting an operatingcondition of the steering wheel and providing an output signal,restoration torque designating means responsive to the output signalfrom the detecting means for calculating a restoration torque signalrepresentative of a restoring torque, independent of vehicle speed, of adirection opposite to rotating directions clockwise andcounterclockwise, respectively of the steering wheel, and summing meansfor summing the assist signal and the restoration torque signal and forproducing a summing signal for actuating the motor, so that steeringangle of the steering wheel converges at a center position betweenclockwise and counterclockwise limits of the steering angle when thesteering wheel is released.